Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by crosslinking of a polymer, heat-induced solubilization, or by particle coagulation of a thermoplastic polymer latex.
Although some of these thermal processes enable plate making without wet processing, the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically comprises an oleophilic binder, e.g. a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in e.g. EP-A 625728, 823327, 825927, 864420, 894622 and 901902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in e.g. EP-A 625,728.
Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer latex, as described in e.g. EP-As 770 494, 770 495, 770 496 and 770 497. These patents disclose a method for making a lithographic printing plate comprising the steps of (1) image-wise exposing an imaging element comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat, (2) and developing the image-wise exposed element by applying fountain and/or ink.
Another plate that works by latex coalescence is described in EP-A 800,928 which discloses a heat-sensitive imaging element comprising on a hydrophilic support an image-recording layer comprising an infrared absorbing compound and hydrophobic thermoplastic particles dispersed in an alkali soluble or swellable resin which contains phenolic hydroxyl groups.
A similar plate is described in U.S. Pat. No. 6,427,595 which discloses a heat-sensitive imaging element for making lithographic printing plates comprising on a hydrophilic surface of a lithographic base an image-recording layer comprising a compound capable of converting light into heat and hydrophobic thermoplastic polymer particles, which have a specific particle size and polydispersity, dispersed in a hydrophilic binder.
EP-A 514,145 and EP-A 599,510 disclose a method for forming images by direct exposure of a radiation sensitive plate comprising a coating comprising core-shell particles having a water insoluble heat softenable core compound and a shell compound which is soluble or swellable in an aqueous alkaline medium. Image-wise exposing with infrared light causes the particles to coalesce, at least partially, to form an image, and the non-coalesced particles are then selectively removed by means of an aqueous alkaline developer. Afterwards, a baking step is performed.
U.S. Pat. No. 6,692,890 discloses a radiation-imageable element comprising a hydrophilic anodized aluminium base with a surface comprising pores and an image forming layer comprising polymer particles coated on the base wherein the ratio of said pores to the average diameter of the polymer particles ranges from about 0.4:1 to 10:1.
EP-A 1,243,413 discloses a method for making a negative-working heat-sensitive lithographic printing plate precursor comprising the steps of (i) applying on a lithographic base having a hydrophilic surface an aqueous dispersion comprising hydrophobic thermoplastic particles and particles of a polymer B which have a softening point lower than the glass transition temperature of said hydrophobic thermoplastic particles and (ii) heating the image-recording layer at a temperature which is higher than the softening point of polymer B and lower than the glass temperature of the hydrophobic thermoplastic particles.
U.S. Pat. No. 5,948,591 discloses a heat sensitive element for making a lithographic printing plate comprising on a base having a hydrophilic surface an image-recording layer including an infrared absorbing agent, hydrophobic thermoplastic particles and a copolymer containing acetal groups and hydroxyl groups which have at least partially reacted with a compound with at least two carboxyl groups.
A problem associated with negative-working printing plates that work according to the mechanism of heat-induced latex coalescence, is to provide both a high run-length during printing and a high sensitivity during exposure. A high run-length can be obtained by exposing the printing plate with a high heat (infrared light) dose—i.e. a high energy density—so that the latex particles in the exposed areas coalesce to a high extent, adhere firmly to the support and are thereby rendered resistant to the development where the non-exposed areas are removed from the support. However, the use of a high energy dose implies a low speed plate which requires a long exposure time and/or a high power laser. When on the other hand a low heat dose is applied, the extent of coalescence is low and the exposed areas degrade rapidly during the press run and as a result, a low run-length is obtained.
Another major problem associated with negative-working printing plates that work according to the mechanism of heat-induced latex coalescence, is the complete and profound removal (i.e. clean out) of the non-exposed areas during the development step. Further problems associated with the development step of printing plates based on heat-induced latex coalescence include the occurrence of flocculation and/or scum during processing and the appearance of stain and/or toning at the non-image areas. During the development step, the non-exposed or non-image areas of the image-recording layer should be removed by the developer solution while the exposed areas or the image-areas should remain essentially unaffected. Thus, not only should the non-image areas be removed thereby revealing the underlying hydrophilic surface of the support, but at the same time the exposed areas should not be affected to such an extent that their ink-acceptance is rendered unacceptable.